This application relates to a sprayable machinable media with good stability. More specifically, it relates to a sprayable gypsum with good machining and spraying properties.
The present invention relates to a plaster mixture for forming a sprayable, machinable composition, and specifically to a plaster mixture that can be adapted to be poured into a mold, trowel applied, or spray applied to a substrate, and machined to a desired shape to produce a dimensionally accurate model.
Gypsum is also known as calcium sulfate dihydrate, terra alba or landplaster. Plaster of Paris is also known as calcined gypsum, stucco, calcium sulfate semihydrate, calcium sulfate half-hydrate or calcium sulfate hemihydrate. Synthetic gypsum, which is a byproduct of flue gas desulfurization processes from power plants, may also be used. When it is mined, raw gypsum is found in the dihydrate form. In this form, there are approximately two water molecules of water associated with each molecule of calcium sulfate. In order to produce the hemihydrate form, the gypsum can be calcined to drive off some of the water of hydration by the following equation:CaSO4·2H2O→CaSO4·½H2O+ 3/2H2O
Calcium sulfate hemihydrate can produce at least two crystal forms. Alpha-calcined gypsum is made by a continuous process or a lump rock process whereby the calcium sulfate dihydrate is calcined under pressure. The alpha-calcined gypsum forms less acicular crystals than beta-calcined gypsum, allowing the crystals to pack tightly together, making a denser and stronger plaster. The crystal morphology allows water to flow easily between the crystals, requiring less water to form a flowable slurry. More elongated crystals are characteristic of the beta-calcined gypsum. This crystal structure results in a less dense product because the crystals are more loosely packed. The beta form also requires more water to fluidize the calcined gypsum. If the calcining of the dihydrate is performed at ambient pressure, the beta form is obtained and the cost is relatively low compared to the alpha-calcined gypsum.
A number of useful gypsum products can be made by mixing the calcium sulfate hemihydrate with water and shaping the resulting product slurry into the desired shape. The product slurry is permitted to set by allowing the calcium sulfate hemihydrate to react with sufficient water to convert the hemihydrate into a matrix of interlocking dihydrate crystals. As the matrix forms, the product slurry becomes firm and holds the desired shape. Excess water must then be removed from the product by drying.
When no additives are used, the amount of water added to a calcined gypsum slurry determines the density of the set gypsum matrix. As more water is used, the slurry increases in volume. The theoretical water demand of pure calcium sulfate hemihydrate is 18.6 wt %. The hydrated calcium sulfate matrix forms filling the volume originally occupied by the slurry, trapping the excess water in the crystal interstices of the gypsum matrix. For equal amounts of gypsum, the interstices are larger and more numerous to take up the excess water as the proportion of water is increased. As the size and number of the spaces increases, both the density and the strength of the matrix decreases compared to a composition with little of no excess water. However, slurries with little excess water are very difficult to mix, particularly if beta-calcined gypsum is used.
The amount of water used determines the plasticity of the slurry. Plasticity is the property of plaster which permits permanent and continuous deformation in any direction. As opposed to low viscosity materials, plastic materials require a measurable force to initiate flow. A material of low plasticity is usually described as being “poor” or hard working; high plasticity is described in terms of “rich” or easy working.
In some types of manufacturing, the molds used in production are cast from models. One method of making a model involves shaping a substrate with CNC or other high performance milling machine. In applications where the models are large, such as in the automobile, watercraft and aerospace industries, a large volume of substrate material is necessary to build a model. Therefore, it is desirable that the substrate material be lightweight and economical. Also, the material must be machinable to a high degree of dimensional accuracy. Accordingly, the material must be able to hold a sharp corner and a smooth contour and be dimensionally stable from room temperature through 250° F. (121° C.).
To produce large models, it is common to first fabricate a substrate that is sized slightly less than the full size model. Inexpensive materials such as tooling boards, Styrofoam and the like are commonly used to fabricate the substrate. The substrate is coated with successive layers of a machinable material until the substrate is larger than the model. Finally, the coated substrate is machined into the desired shape.
Plaster mixtures for forming machinable compositions are known. Notably, U.S. Pat. No. 5,534,059 to Immordino describes a high density gypsum composition for use as a machinable composition, which is readily carvable, is able to hold sharp corners and smooth contours, and is dimensionally stable from room temperature to 250° F. (121° C.). This plaster contains a polymer binder for increasing strength of the plaster and for preventing the generation of dust during machining. As disclosed in the Immordino patent, the binder is an ethylene/vinyl acetate/vinyl chloride terpolymer. Up to 10 to 20 percent by weight of polymer binder is preferably added to the plaster to obtain the desired consistency. The composition of Immordino discloses the addition of an internal lubricating agent to minimize friction during machining. Further, this composition may not be spray applied to a substrate since, if sprayed, due to the effects of gravity, the composition will flow prior to set and before building up a machinable thickness.
U.S. Pat. No. 6,355,099 to Kaligian et al discloses a sprayable, machinable plaster utilizing an internal binder, an external binder and a system of accelerators and retarders. Although a proper set time is obtainable, the accelerator system is complex and is sensitive to temperature and other conditions.
Machinable materials adapted to be spray applied on a substrate are also known, namely, polyester and polyurethane foams. Polyester is undesirable because it is possible to apply only about a 1/16 inch (1.6 mm) coat at a time. Therefore, numerous coats of polyester are needed to build up an appreciable thickness. Also, because each coat must be allowed to dry before the next coat, this method is time consuming.
Polyurethane foams are undesirable because they do not produce a coat with a uniform thickness and the coats normally have numerous air pockets. Therefore, to achieve dimensional accuracy, significant surface preparation is necessary following machining. Polyester and polyurethane foams generate static electricity and undesirable amounts of dust during machining. The combination of the static and the dust creates an unwanted mess. Further, the isocyanate used for curing the polyurethanes is a known carcinogen.
Thus, there is a need in the art for a means to stabilize the set time of a slurry and to allow the sprayed composition to build up quickly to a machinable thickness. There is a further need to increase the concentration of beta-calcined gypsum relative to alpha-calcined gypsum at approximately a constant water demand. When these needs are met, products made of sprayable, machinable plasters can be made more quickly, with less waste and/or at lower cost.